> which 'costs' only 75 microWatt per message
Typical ignorant nonsensical misuse of units.
Maybe they mean 75 micro Joules per message. Or maybe they mean it consumes 75 micro Watts while sending messages (taking how long?). But who knows; it’s like saying “the shop is 30 mph away” or “the train’s top speed is 125 miles” or “this wind farm produces enough energy to supply a city the size of Bristol each day”.